The history of the corn tortilla is lost in the pre-Columbian time with the ancestors of the native Aztec and Maya peoples. The basic steps in making tortillas has continued unaltered since these times. In fact, the traditional process employed by the first inhabitants of Mesoamerica is still been used today.
Corn tortillas are one of the most important food products of Mexico and of some Central American countries. It constitutes the major source of proteins and calories for the lowest social economical groups. At the present time, there is a large expansion in the consumption of tortillas in the U.S.A. as well. Tortillas are also employed as the raw material for the preparation of innumerable traditional food products such as snacks, tortilla chips, taco shells, tostadas, enchiladas, burritos and nachos, all of which are extremely popular in the southern states of the U.S.A. In Mexico and some Central American countries, tortillas, under various types of products, constitute a significant part of the diet of every social strata, with the average annual per capita consumption of 182 Kg. In rural areas they provide approximately 70% of all calories and 50% of all proteins ingested daily.
The traditional method of nixtamalization consists in the alkaline cooking of whole corn in water to which 1 to 2% by weight of lime has been added, by heating the mixture to boiling for 30-40 min. and steeping the cooked mixture for 12 to 15 hr. The steeped liquor rich in solids (called nejayote) is discharged. The cooked steeped maize (called nixtamal) is washed to remove excess of lime and lost pericarp tissue. The nixtamal is ground with a pestle and stone into dough (called masa) and flattened into thin disks that ape baked for 30-60 sec on each side on a hot griddle (comal) heated to 180.degree. to 210.degree. C.
In industrial processes corn is cooked using several different cooking and steeping processes. Important process variables are: the type of corn, the cooking time and temperature, the lime concentration, the agitation frequency (which is necessary to keep the lime suspended) and the nixtamal washing procedures. A common procedure for cooking corn uses steam kettles which contain water, lime and corn, into which steam is injected until a temperature near boiling is reached. The cooking water is circulated and the suspension is stirred to suspend the lime. To be used for the manufacture of tortillas the corn is held at near boiling for 30 to 40 minutes; for other uses, such as the manufacture of tortilla chips less time is required.
After cooking, cold water is added and the mixture is steeped for 8 to 12 hr. Afterwards the nixtamal is washed generally using horizontal rotating barrels or drums with the addition of pressurized water in the form of a spray. For the production of fresh masa the nixtamal is washed again and then milled. To prepare tortillas the masa is shaped into circular pieces and baked, generally in a gas oven with conveyor belts heated to a temperature of between 300.degree. and 320.degree. C.
The traditional commercial process for the production of tortillas and dry masa flours described above as admitted prior art involves excessive time and large floor space, as well as the production of liquid waste. In addition, as a result of the process important nutrients such as thiamin, niacin, riboflavin, fat and fiber are lost. The liquid waste from the cooking process, or nejayote, contains solid particles consisting of fragments of the pericarp (fiber), starch, protein and soluble germ. In addition to the above drawbacks, in some cases subjective determinations and empirical methods used in each step of the process may affect the quality of the final product. All these factors have important economical and commercial implications.
Several research works had been conducted with the objective of making the traditional nixtamalization process more efficient. These mainly consider relationships between concentration of water, corn and lime, the cooking temperature and time, the steeping time of the cooked corn and its milling, without modifying the basic characteristics of the traditional nixtamalization process.
Such works have been described, for instance, in U.S. Pat. No. 2,584,893, patented on February 1952 to Lloyd, W. R. and Millares Sotres, R., which discloses a method for making tortilla flour that includes a stage that comprises the traditional nixtamalization process described above.
U.S. Pat. No. 2,704,257, patented on Mar. 15, 1955 to Diez de Sollano, C. S. F. and Berriozabal, J. M. discloses a method for producing corn tortilla flour which includes an improvement over the traditional nixtamalization process, by directly converting the wet nixtamal into a dry flour in one single milling and drying step. The traditional cooking and steeping steps, however, continue to be carried out prior to the milling and drying of the nixtamal corn grains, although the steeping step is carried out by the use of perforated drums within which the corn kernels are contained, said drums being immersed into a steeping bath containing the calcium oxide. The main advantage of this method is that of omitting the obtention of a corn dough or masa for further transformation into a flour. The drawbacks shown by the ancient cooking and steeping steps, however, remain substantially unchanged.
Anderson, E. E. and Brown, J. D. describe in U.S. Pat. No. 3,083,103, patented on Mar. 26, 1963, a method that allegedly reduces the treatment time for corn kernels to one tenth as compared with the traditional nixtamalization process, However, Anderson et al use a supersaturated calcium hydroxide solution as the steeping liquor and carry out the steeping operation under high pressure, which obviously require a special type of costly equipment. On the other hand, the supersaturated alkaline steeping bath renders the control of the alkalinity rather difficult and frequently produces an excessively alkaline nixtamal which will create an off-flavor in the finished product. Therefore, this method has not gained popularity in the market.
Eytinge, B. D., U.S. Pat. No. 3,194,654 patented on Jul. 13, 1965, discloses a method for the production of nixtamal which, without departing from the traditional nixtamalization process, renders said process continuous by providing a steeping receptacle at the top of which the corn kernels are continuously fed and at the bottom of which an alkaline steeping solution is fed in counter-current with said kernels and at a suitable cooking temperature to accomplish the nixtamalization of the corn in a time of approximately 5 hours. Although this method represents an improvement over the traditional nixtamalization process, it requires the use of very costly equipment that is difficultly controllable.
U.S. Pat. No. 3,404,986, patented on Oct. 8, 1968 to Wimmer, E. L., Grove, E. and Susek, J. L., discloses a method for producing corn flours suitable for fried snacks, but which would be absolutely unsuitable for the manufacture of tortillas and the like. Wimmer et al use a mechanical method for separating the germ, endosperm and hull of the kernel and then prepare mixtures of whole corn meal and endosperm enriched meal and subject it to the action of hot steel rollers to cause starch gelatinization and drying of the meal. Although a masa flour is obtained by this process which can be used for fried snacks, it is not useful for the manufacture of tortillas in view of the absence of an alkaline stooping step which will soften the masa flour as is necessary for making tortilla flour.
Celorio Mendoza, F., U.S. Pat. No. 3,859,452, patented Jan. 7, 1975, describes a method for nixtamalizing whole corn flours by passing a mixture of said flour and lime upwardly through a vertical treating zone by means of an ascending air stream, heating the mixture to release steam from the flour, thus wetting the lime in order to react with the flour and nixtamalize the same, and passing the humid air entrained mixture to a cooling zone to cause reabsorption of the water. This variation of the traditional nixtamalization process, as it may be easily seen, lacks means for controlling the uniformity of the nixtamalization of the treated flour and therefore is incapable or producing a nixtamalized flour with sufficient uniformity for use in the manufacture of tortillas.
Hart, E. R., U.S. Pat. No. 4,329,371, patented on May 11, 1982, describes a method of obtaining corn masa without the use of the alkaline nixtamalization, by providing a step of dehulling the corn grains by means of an extremely complex hydraulic dehulling apparatus, separating the hull from the endosperm and germ portion of the grain, using the hull for other applications, and cooking the endosperm and germ portion with steam. Although this method is capable of obtaining a masa flour perfectly suitable for the manufacture of tortillas, it shows the drawback that the hull must be discarded or used for other purposes of lower economical value.
Tonella, M. L., Sanchez, M. and Salazar, M. G., in Journal of Food Science, Volume 48, 1983, describe on page 1637 a method for obtaining nixtamalized corn flour which uses a slight modification of the traditional nixtamalization process by cooking the suspension of corn and lime at a slightly higher temperature and steeping the suspension for about 24 hours, to then rinse with water and dry the kernels in a tunnel drier and mill the dried kernels in a Ciclotex mill. This method does not depart from the traditional nixtamalization process and therefore does not represent an advance in the art.
U.S. Pat. No. 4,513,018, patented on Apr. 23, 1985 to Rubio, M. J., discloses a continuous method for producing corn flour suitable for making tortillas, taco shells, tortilla chips and the like, which uses a modified nixtamalization process comprising the precooking of corn grains in the presence of lime within a receptacle provided with a screw conveyor. The precooked kernels are then treated with a spray of hot water and are passed to a cooling zone where they remain for a time sufficient to reabsorb a sufficient amount of water. The thus steeped grains are milled and the flour is classified and re-milled until the desired particle size is obtained. This method, although representing an improvement over the traditional nixtamalization process, still must use the same by cooking the grain in an alkaline bath for a considerable period of time.
Vaqueiro, H. C. and Reyes, P. disclose, in U.S. Pat. No. 4,594,260, patented on Jun. 10, 1986, a method of obtaining corn flour including a particular nixtamalization stage which essentially comprises removing the hull from the germ and endosperm of the corn kernel by humidifying, crushing and sieving the integral corn kernels until a fraction containing hull and a fraction containing endosperm and germ are obtained, and then nixtamalizing by the traditional method only the hull fraction and re-mixing the same with the non nixtamalized endosperm and germ fraction. This method, although more efficient than the traditional methods in view of the fact that only a small fraction of the corn kernel has to be nixtamalized, requires a rather elaborate equipment and takes a relatively long time to effect full nixtamalization of the hull fraction,
As it may be seen from the above, the prior art methods for the production of corn dough or masa involve excessive time and large floor space, as well as the production of large amounts of liquid waste. In addition, important nutrients such as thiamin, niacin, riboflavin, fat and fiber are lost.
With the aim of solving the above described problems shown by the traditional nixtamalization process and the modifications thereof, some workers have developed certain manners of cooking by extrusion cereal grains. Thus, Matsumoto, Y. and Taguchi, G. describe in U.S. Pat. No. 4,748,037, patented on May 31, 1988, a cooker-extruder which utilizes the grains as they are, without a previous crushing step, and treats said grains, under controlled humidity conditions, by means of a twin-screw extruder under a relatively high temperature and expelling the crushed cooked grain through a die. Although corn kernels are mentioned as one of the cereals which may be treated in this cooker-extruder, the description is not enabling towards said grains, and it may be assumed that the corn grains must be used after a dehulling operation is carried out, because the conditions of cooking and extrusion would not be able in themselves to nixtamalize the hulls of corn grains.
In U.S. Pat. No. 4,756,921, patented on Jul. 12, 1988 to Calandro, T., Straks, R. and Verrico, M., a similar twin-screw cooker extruder is described, in which different types of cereal brans are used, including corn (maize) bran. This cooker-extruder works under high shear forces which unduly heat the material under treatment, which obviously brings about difficulties in the control of the process in order to avoid scorching, and is generally intended for treating mixtures of bran and syrup by means of a high shear blending zone, a cooking zone and a cooling zone, from which the paste is discharged through a die having perforations predetermined size and shape in order to directly form cereal nuggets.
U.S. Pat. No. 4,769,251, patented on Sep. 6, 1988 to Wenget, M. L. and Huber, G. R. describes a cooker-extruder specially designed for obtaining cooked rice products and comprising a preconditioning zone where the rice grains are precooked, a cooking zone where the mass of grains are heated, a vent or vacuum zone to apply a vacuum to cool the rice, and a forming zone containing a die of predetermined size and shape to obtain an extruded rice product. This cooker-extruder, however, is not suitable for the treatment of corn in view of the rather special characteristics thereof.
Finally, U.S. Pat. No. 4,985,269, patented on Jan. 15, 1991 to Irvin, S. A., Foder, R. A. and Merritt, C. G., describes a twin-screw high shear cooker extruder to which corn kernels and lime-water are fed. The extruder contains a first section where mixing and blending is effected under low shear and low compression, a second section which is one of intense mixing, shearing and kneading and where cooking is carried out by the provision of heat mainly originated from the high shear forces to which the dough is subjected, supplemented by heat from an external source, and a third low shear section where the dough is cooled prior to discharge through the die of the extruder in order to avoid expansion of the dough after leaving the die. This extruder is highly suitable for the production of a rather compact, non expanded dough useable for the preparation of tortilla chips, but which could hardly be used for the production of puffed tortillas.
Referring in general terms to the above discussed prior art cooker-extruders, it is to be noted that the screw is probably the most important part of the extruder as it governs not only the degree of cooking and gelatinization but also the quality of the final product. The most drastic changes in the cooking-extrusion process are due to changes in the geometry of the screw. The performance, efficiency and capabilities of an extruder depend to a large extent on the design of the screw and the screw sleeves which govern the flow mechanism of the material in the extruder. Generally speaking, in the prior art extruders mentioned above, the screw is divided into several zones with differing pressure and with a compression ratio higher than 1, and where the shear and pressure are increased in order to accelerate the cooking of the product.
Although the above described prior art cooker-extruders are capable of producing either-instant corn flour or instant corn dough or masa in very short times and with a quality suitable for the obtention of tortilla chips and the like, they are not suitable for obtaining a soft dough having the special characteristics necessary to produce table tortillas or even taco shells, whereby an extruder capable of producing such a nixtamalized corn dough or masa was still unavailable in the market.